Second-order drift-kick-drift step ported from REBOUND integrator_leapfrog.c, copyright Hanno Rein and Shangfei Liu, GPL-3.0-or-later. New 2D restricted two-core gravity model and interface; tracer stars do not self-gravitate. Dimensionless units, force softening 0.2, fixed step 0.005. Full referenced upstream integrator and editable browser source included.
Repository https://github.com/hannorein/rebound
Commit 262a9cce102c69ca2cf7f3c6c1264e5e60002b7d
Complete editable adaptation: all local HTML/CSS/JS files. No build required.

Source lineage: original interactive mission design around translated computation; upstream project is a scientific C library, not a browser-native galaxy game. Targeted browser-source searches found no qualifying maintained licensed app for this exact experiment; this port uses only the disclosed algorithms. Current relevant inspiration and research are preserved in worker evidence.